In recent years, in order to solve the waste disposal problem, various wastes are used as a part of a cement raw material or a part of a heating fuel used in a cement kiln. However, when wastes, such as synthetic resin, are charged into the cement kiln as a part of the fuel, a chlorine component having volatility is generated during combustion. The chlorine component is sent to a preheater together with exhaust gas discharged out of the cement kiln. However, when the atmosphere temperature becomes below the melting point of the chlorine component as the exhaust gas is transferred to the upper stage side of the preheater, the chlorine component is condensed to adhere to the cement raw material, and is again sent to the inside of the cement kiln, so as to be evaporated again according to the increase in the atmosphere temperature.
In this way, the chlorine component taken into the system of the cement burning facility is circulated by repeating evaporation and condensation in the inside of the cement kiln and the preheater. Also, when a chlorine component generated from newly charged wastes is added to the circulating chlorine component, the concentration of the chlorine component is increased, so as to thereby result in clogging in the preheater due to coating, and the like. For this reason, there arise problems that stable operation of the facility is obstructed and that the quality of manufactured cement clinker is also affected.
Thus, in order to solve the above described problem, for example, in patent document 1 as will be described below, there is proposed a kiln exhaust gas treatment method based on the chlorine bypass, which method includes: a process of extracting a part of kiln exhaust gas from a kiln; a process of cooling the extracted exhaust gas to the melting point or lower of a chlorine compound; a process of separating dust in the exhaust gas into coarse powder and fine powder by a classifier; and a process of returning the separated coarse powder to the kiln and of discharging the fine powder to the downstream side of the classifier, and which method is characterized in that the ratio of extraction amount of the kiln exhaust gas is set to more than 0% to 5% or less, in that the separation particle size in the classifier is set to 5 μm to 7 μm, and in that the discharged amount of the fine powder is set to 0.1% or less of the production amount of the clinker.
According to the kiln exhaust gas treatment method having the above described configuration, it is possible to stably operate the rotary kiln in such a manner that the exhaust gas, which contains the fine powder dust separated in the classifier and having a high chlorine content, is sent to a dust collector, and that the fine powder dust having the high chlorine content is collected and discharged to the outside of the system. Further, it is possible to effectively remove the chlorine at a minimum heat loss and possible to reduce the extracted gas amount, and hence the scale of the treatment facility can be reduced. As a result, it is possible to obtain the effects that the space and cost of the facility can be reduced, and that the kiln can be economically and stably operated.
However, in the conventional kiln exhaust gas treatment method as described above, the particle diameter of the fine powder dust collected by the dust collector is set to a very fine particle diameter in the range of 7 μm down to 5 μm or less. For this reason, a general purpose type cannot be used as the classifier, and hence it is necessary to install a classifier having high performance, which needs to be finely controlled during the operation. This results in a problem of increase in the installation cost of the classifier.
In addition, the very fine powder dust is collected, so that the chlorine concentration in the collected fine powder dust is extremely increased. For this reason, in the case where the chlorine concentration in the fine powder dust exceeds 20%, the handling property of the collected fine powder dust is significantly deteriorated due to the deliquescence, or the like, of the chlorine compound adhering to the fine powder dust. This also leads to a problem that the fine powder dust adheres to the inner peripheral wall of a conveying pipe, a hopper, and the like (to cause the coating), so as to easily cause a trouble such as clogging of hopper and chute. As a result, there are problems that it is difficult to perform the stable operation and that the maintenance cost is also increased.
Patent document 1: Japanese Patent No. 3318714